Single-walled carbon nanotubes (SWNTs) and their composites are ideal candidates for chemical and biological sensing application, due to their unique electronic, chemical and physical properties. Green tea, or more specifically its main antioxidant component, epigallocatechin gallate (EGCG), has been found to disperse SWNTs in water. However, the chemical sensitivity of this SWNT/green tea (SWNT/EGCG) composite has not been explored. With the presence of EGCG, this SWNT composite should have strong antioxidant properties and thus respond to reactive oxygen species (ROS). Here we report the fabrication and characterization of a SWNT/Green Tea (SWNT/EGCG) sensing system for the detection of H2O2, in both vapor and solution phase. It was observed that the conductance of SWNT/EGCG composite thin film increased with increase in H2O2 concentrations. We further investigated the sensing mechanism by Fourier-transform infrared (FTIR) spectroscopy and propose here that the response to H2O2 arises from the oxidation of EGCG in the composite. These results were further validated using liquid-gating field-effect (FET) transistor measurements. These findings suggest that SWNT/Green Tea composite has a great potential for developing simple resistivity-based nanoscale sensors for the electronic detection of ROS.